Natural resources such as gas, oil, and water residing in a subterranean formation can be recovered by drilling wells into the formation. Well drilling involves drilling a wellbore down to the formation while circulating a drilling fluid or mud through the wellbore. Various types of drilling fluids, also known as drill-in fluids when used in the productive interval, have been used in well drilling, such as water-based fluids, mineral oil-based fluids, and synthetic oil-based fluids. Such drilling fluids form a thin, slick filter cake on the formation face that provides for successful drilling of the wellbore and that helps prevent loss of fluid to the subterranean formation.
Several stages may be used to produce oil found in subterranean formations. The first stage, which is known as the primary production stage, allows the oil to flow into a production well (or wells) under natural forces. At first, the natural forces may be sufficient to drive the oil to the surface where it is recovered. However, at some point, pumps may be required to displace the oil from the wellbore to the surface. The primary production stage usually yields only about 5% to 15% of the oil in the reservoir. A secondary recovery operation thus is typically performed to recover additional amounts of the oil from the reservoir. A common secondary recovery operation known as secondary flooding involves injecting a fluid such as water into a so-called injection well (or wells) to drive oil in the formation to the production well (or wells). Secondary flooding usually recovers up to an additional 50% of the original oil in the reservoir. Tertiary recovery operations such as tertiary flooding may also be used to drive the remaining oil from the formation to the production well. Unfortunately, the presence of the filter cake on the face of the subterranean formation can adversely affect the flow of fluid though the injection wells and the production wells. In the case of the injection wells, particularly in deepwater environments, the injected fluid is not flowed back to remove the filter cake left by the drill-in fluid. The pump pressures (e.g., fracturing pressures) required to inject past the filter cake are higher than desirable for achieving good sweep efficiency of the oil.
Many subterranean formations are unconsolidated or poorly consolidated. Thus, loose sand grains may undesirably flow into an adjacent production well, contaminating the fluid being recovered from the well. The sand could cause severe erosion of well equipment and could plug the flow passages into the well such that an expensive workover of the well is required. One method commonly utilized to prevent migration of sand into wells and to maintain the integrity of subterranean formations is a well completion method known as gravel packing. In gravel packing, a permeable screen is placed against the face of a subterranean formation, followed by packing gravel against the exterior of the screen. The size of the gravel particles used for this purpose are larger than the sand particles but are also small enough to ensure that sand cannot pass through voids between the particles. The gravel is typically carried to the subterranean formation by suspending the gravel in a so-called gravel packing fluid and pumping the fluid to the formation. The screen blocks the passage of the gravel but not the fluid into the subterranean formation such that the screen prevents the gravel from being circulated out of the hole, which leaves it in place. The gravel is separated from the fluid as the fluid flows through the screen leaving it deposited on the exterior of the screen. Fluid leakoff into the formation matrix can also result in the sand being placed around the screen.
Once an open hole wellbore or interval has been drilled to penetrate a subterranean formation, the filter cake cannot be removed from the face of the formation before gravel packing the formation. Otherwise, an excessive amount of fluid containing sand could pass from the formation into the wellbore while attempting to remove the filter cake. Horizontal wellbores are particularly limited in this respect because they are typically thousands of feet in length and thus have a relatively large surface area through which fluid can pass. Further, using an acid wash to remove the filter cake after the gravel has been placed in the well is usually not an option due to the excessive cost involved combined with the inherent difficulty in placing acid uniformly across the entire interval without excessive leak-off of the acid into the localized areas along the wellbore, particularly in horizontal completions. In post drilling acidizing, an additional risk of damaging the formation from the spent or partially spent acid leaking off into the formation exists. A need therefore exists for relatively inexpensive methods of removing a filter cake from a subterranean formation without risking the loss of substantial amounts of potentially damaging fluid into the formation.